1. Field of the Invention
This invention relates to a process for separation of water and cyclic hydrocarbons from a gas mixture containing hydrogen, methane, water and cyclic hydrocarbons, such as may be usefully employed for prepurification of effluent gas from a toluene dealkylation process prior to cryogenic separation of the gas mixture.
2. Description of the Prior Art
In a typical toluene dealkylation (TDA) process, a toluene feed stock is mixed with a compressed stream of excess high purity hydrogen and heated to high temperature. The resulting gaseous mixture is then reacted at high temperatures and generally high pressures, either with or without catalysis, to produce an effluent gas containing benzene, methane and unreacted hydrogen and toluene.
Regardless of whether or not catalysis is employed, the method of recovery and purification of the effluent gas from the TDA process is reasonably standard. The effluent gas from the TDA process is initially cooled, first by heat exchange with the feed stream to the toluene dealkylation process and then by supplemental refrigeration, normally a cooling water stream. The resultant two-phase gas mixture thus produced is then phase separated at about 100.degree. F. to yield a benzene-rich liquid and a methane- and hydrogen-containing vapor. The benzene-rich liquid is processed in a recovery and purification system, for upgrading the benzene purity of the liquid. The methane- and hydrogen-containing vapor is concurrently processed to reject methane and return a hydrogen stream to the toluene dealkylation process.
The methane rejection-hydrogen recovery process just mentioned has conventionally involved cryogenic processing of the methane- and hydrogen-containing vapor. The major advantages of cryogenic processes include extremely low energy requirements and relatively low investment costs as compared to other competitive processes, such as absorption systems. The utilization of a cryogenic process, however, requires careful preparation and purification of the feed vapor to remove freezable components. In addition to water (and in many cases hydrogen sulfide), the feed gas mixture for the cryogenic process from the crude benzene separation step is saturated with benzene and toluene. Due to the high freezing point of benzene, plus 42.degree. F., benzene must be removed from the gas mixture along with water and any other contaminants prior to processing in the cryogenic system. The prior art approach to such benzene removal has involved a toluene wash process.
In the conventional toluene wash prepurification process, feed gas mixture for the cryogenic unit, which comprises the vapor fraction from the crude benzene separation described above, is countercurrently flowed against a high purity toluene wash liquid in a gas-liquid contacting column. An impure toluene bottoms liquid is recovered from the column which then augments the feedstock for the toluene dealkylation process, while the overhead vapor recovered from the column, which is saturated with toluene instead of benzene, passes to the cryogenic unit. Since the freezing point of toluene is minus 139.degree. F., it does not pose as severe a threat to the operation of the cryogenic system as does benzene. Most of the toluene can be removed by condensation after an initial cooling step in the cryogenic system; concomittantly, a substantial portion of the water is also removed from the gas mixture. The uncondensed vapor from which condensed toluene has been removed is then passed through an adsorbent dryer prior to further cryogenic processing. In conventional practice, two adsorption beds are provided for drying (residual water removal) so that while one is processing feed gas mixture, the other is being regenerated at lower pressure with a heated portion of the methane rejected in the cryogenic system.
Although the above-described toluene washing operation has been successfully practiced in conjunction with cryogenic treatment of toluene dealkylation effluent gas, it suffers from several potential disadvantages. First of all, to maintain suitable benzene removal, the toluene wash liquid must be very pure, typically containing no more than 2.0% benzene by volume. Since the toluene feed purity specifications for a normal toluene dealkylation process may be as low as 90%, with benzene the major impurity, high purity toluene may not be available in the process system, and auxiliary equipment may be required to produce such a stream. Secondly, even though a substantial quantity of benzene can be removed from the toluene dealkylation effluent gas by toluene washing, there may still be a problem with residual benzene freezing in the heat exchangers of the cryogenic processing system. This is because benzene cannot be removed in the dryers of the adsorption zone, inasmuch as conventional dryers utilize a molecular sieve adsorbent which will not remove a significant quantity of aromatics.
As an alternative to the toluene washing process, it is known to employ an adsorption system for treatment of the effluent gas from the toluene dealkylation unit. Such an adsorption system replaces the toluene wash column, and associated equipment and the adsorbent dryers of the toluene wash system. As a result, the adsorption system exhibits a high potential for economic savings. Such an adsorption system consists of at least two compound adsorbent beds, each containing two adsorbent zones, arranged in parallel flow relationship, such that while one bed is processing the toluene dealkylation effluent gas mixture, the other bed is being regenerated for subsequent gas mixture processing. Typically, the first zone in the compound adsorbent bed contains silica gel adsorbent for aromatics removal while the second zone contains molecular sieve adsorbent for water removal.
In operation of the adsorption system, the methane- and hydrogen-containing vapor phase recovered in the aforementioned crude benzene separation operation is cooled with refrigeration to a temperature in the vicinity of 40.degree. F., phase separated at a temperature as close as possible to the freezing point of benzene and the resulting gas phase is alternately passed through the on-stream adsorbent bed in the adsorption system. Following the adsorption step, the adsorbate-loaded adsorbent bed is thermally regenerated by countercurrently passing a heated portion of the product hydrogen stream from the cryogenic process therethrough. This regeneration stream can then be recycled directly to the toluene dealkylation unit as a portion of the hydrogen requirement therefor, or, alternatively, this stream may be cooled prior to recycling to condense a portion of its aromatic constituents.
Although the adsorption system may, in many cases, provide a suitable replacement for the toluene wash system, the former is plagued by several potential disadvantages. For example, in addition to water, methane, hydrogen and aromatics, the effluent gas mixture from a toluene dealkylation process may also contain appreciable amounts of hydrogen sulfide. Hydrogen sulfide can appear in the toluene dealkylation process effluent gas mixture from primarily two sources. First, sulfur compounds may be present in the make-up hydrogen feed stream for the toluene dealkylation unit; the toluene dealkylation reactor generally converts all such compounds to hydrogen sulfide. Since, when catalysis is used, the toluene dealkylation process is sensitive, in varying degrees, to the presence of hydrogen sulfide, such catalytic processes may be provided with a make-up gas pretreatment step to minimize the source of hydrogen sulfide. Second, carbonyl sulfide may be purposely added to the feed gas streams for a thermal toluene dealkylation reactor as a corrosion inhibitor. Subsequently, in the toluene dealkylation reactor, the carbonyl sulfide is converted to hydrogen sulfide.
Any hydrogen sulfide in the toluene dealkylation effluent gas (vapor fraction recovered from the crude benzene separation operation) will be primarily coadsorbed with water in the adsorption system. This hydrogen sulfide is subsequently recycled with the aromatics to the toluene dealkylation unit after regeneration of the adsorbent beds, by hot hydrogen purging thereof. The recycled hydrogen sulfide will add to any sulfur compounds present in the toluene dealkylation reactor feed gas and increase the dealkylation loop's overall sulfur content. Repetitive adsorption-desorption of hydrogen sulfide in the adsorption system may continue for a lengthy period as a result of the adsorbent's ability to co-adsorb fairly large quantities of hydrogen sulfide. As a result, hydrogen sulfide will build up until a sufficient quantity is present in the dealkylation loop to either poison the dealkylation reactor catalyst, in the case of a catalytic toluene dealkylation system, or else saturate the adsorbent zone and subsequently break through in sufficient quantities to freeze the cryogenic system.
It is also known that a heated stream of rejected methane from the cryogenic system can also be used as the regeneration gas for regeneration of the adsorbent beds in the adsorption system. In this manner, any hydrogen sulfide which is coadsorbed with water will be rejected to fuel with the methane-rich gas, thereby preventing buildup of hydrogen sulfide in the dealkylation loop (of recycled hydrogen-rich gas). Nonetheless, such an approach results in significant aromatics losses via the rejected fuel gas purging stream (methane-rich gas). Recovery of aromatics from the adsorption system regeneration gas requires the added expense of an additional refrigeration and recovery step. In addition, only about 50% of the adsorbed aromatics can be easily recovered, resulting in only 85% to 91% overall recovery of aromatics from the gaseous phase obtained from the crude benzene separation, as compared to 99% recovery levels typical for the toluene wash system. However, the adsorption system still offers a simpler and more reliable pretreatment than the conventional toluene wash system.
In addition to processing the toluene dealkylation effluent gas mixture for the recovery of recycle hydrogen, the cryogenic system can also be employed to purify the hydrogen make-up stream for the dealkylation unit. This integrated operation is in many cases more economical than separate treatment. Sources of hydrogen for toluene dealkylation include, among others, catalytic reforming, ethylene plant off-gas and coal gasification; such processes produce gases containing water, carbon dioxide, carbon monoxide and light alkyl hydrocarbons in addition to the desired hydrogen. The cryogenic system can safely handle the carbon monoxide and light hydrocarbons, but before processing such a stream, the water content, carbon dioxide content and other potential contaminants such as hydrogen sulfide must be removed therefrom. In convention practice, this is normally accomplished by passing the make-up hydrogen stream through a bed of adsorbent material which has an affinity for water, carbon dioxide and the other contaminants, as for example, molecular sieve. In this manner the make-up hydrogen stream may be conveniently processed, together with the toluene dealkylation effluent gas mixture (vapor phase recovered from the crude benzene separation), in the adsorption system. However, as is the case when processing an effluent gas mixture stream containing hydrogen sulfide, regeneration of the adsorption system with a hot stream of recycled hydrogen gas derived from the cryogenic system allows the concentration of impurities, e.g., water, carbon dioxide and the other contaminants in the make-up hydrogen stream, to build up in the dealkylation loop. Eventually, the adsorption unit will become saturated with these impurities and they will break through the adsorption system in quantities sufficient to freeze the cryogenic system. To prevent such occurrence, the adsorbent beds in the adsorption system must be regenerated in the same manner as when treating a hydrogen sulfide-containing effluent gas mixture, i.e., regeneration with hot fuel gas (rejected methane) from the cryogenic system, even though such regeneration leads to low aromatics recovery. Another problem associated with processing the toluene dealkylation effluent gas mixture and hydrogen make-up streams together in the adsorption system in that the presence of higher aliphatic hydrocarbons in the hydrogen make-up stream will tend to lower the loading of aromatic constituents such as benzene on the adsorbent in the beds of the adsorption system, thereby reducing the recovery of such aromatic constituents from the toluene dealkylation effluent gas mixture.
In addition to the above-mentioned disadvantages, mixing the hydrogen make-up stream with the toluene dealkylation effluent gas stream for concurrent introduction to the adsorption system results in an increased adsorbent requirement in the adsorption system for removal of aromatic constituents from the combined gas stream. In order to carry out the concurrent introduction of toluene dealkylation effluent gas and hydrogen make-up gas to the adsorption system in the toluene dealkylation system, the hydrogen make-up stream could merge with the toluene dealkylation effluent gas either prior to or subsequent to the low temperature benzene separation, which is conventionally conducted at 45.degree. to 50.degree. F. In the first case, where the gas streams are merged prior to the benzene separation step a greater quantity of gas is recovered from the low temperature benzene separation, but since any benzene in the vapor phase is still at its appropriate saturation pressure, a larger absolute quantity of benzene is present in the vapor phase. Thus, a larger quantity of adsorbent is required than is necessary when processing only the effluent gas stream from the toluene dealkylation unit. In the second instance, the addition of hydrogen make-up gas to the vapor phase recovered from the low temperature benzene separation dilutes the benzene concentration of the separated gas and may necessitate increased adsorbent requirements, as a result of the reduction in benzene partial pressure.
Accordingly, it is an object of the present invention to provide an improved process for the separation of water and cyclic hydrocarbons from a gas mixture containing hydrogen, methane and the water and cyclic hydrocarbons.
It is a further object of the present invention to provide a method for preventing undesirable gaseous components such as H.sub.2 S and CO.sub.2 from fouling behavior in the toluene dealkylation system, as a result of their build-up in the dealkylation loop, and from causing freeze-up of the cryogenic system, while simultaneously obtaining high recovery of cyclic hydrocarbon (aromatic) constituents from the toluene dealkylation effluent gas mixture.
Other objects and advantages of the present invention will be apparent from the ensuing disclosure and appended claims.